This invention relates to transfer printing of sheet metal and in particular to a method and apparatus for transfer printing an image on the coated sheet metal. Transfer printing such as sublimation printing transfers dyes or inks into the clear coating on the metal by pressing transfer paper against the clear coated surface under heat and pressure.
Transfer printing of inks or dyes into a variety of articles such as shirts, mugs, plastic articles and plastic coated substrates is well-known in the art. Dispersible dye crystals or inks are printed in reverse images in transfer paper which is to be used to transfer print the article to be decorated. The article to be decorated is usually made of plastic or has a plastic or polymer coating on it into which the dyes are transferred.
Most transfer printing is referred to as sublimation printing in which the dyes are said to sublimate under heat and pressure to be driven into any receptive substrate that is put in contact with the transfer paper. Such sublimation printing was developed circa 1969 and has been used extensively to print many articles including plastic coated metal substrates.
Transfer printing also includes a melt printing process which is as described in several patents and patent applications including U.S. Pat. Nos. 4,587,155; 4,670,084; 4,668,239 and Published Application WO 92/21514. According to U.S. Pat. No. 4,587,155, the desired dye image is transferred from the paper to the substrate by heating the dye to a temperature above its melting point but below its vaporization temperature so the dye will diffuse into the softened plastic substrate. Published Application WO 92/21 514 describes melt printing of planar metal base members such as aluminum, steel or the like which have been coated on at least one planar surface with a melt printable layer of softenable, dye-permeable, thermoplastic or thermoset material such as polyethylene terephthalate, polybutylene terephthalate or other thermoplastic polyesters, polycarbonates, nylons and the like. Application WO 92/21514 further describes bilayer coatings of thermoplastic or thermoset materials including a base coat optimally provided with a pigment and a second layer that can comprise a clear resin.
Canadian Patent 1,108,929 describes a process for applying designs to unsealed, anodized aluminum which is continuously heated to a temperature at which colored components of ink will sublime. The heated strip is ted into contact with an ink carrying web so the ink is heated and transferred into the anodized aluminum surface. The decorated aluminum strip is then cooled with water and passed through a sealing bath filled with a sealing solution such as buffered aqueous nickel.
A variety of machines are commercially available for transfer printing textiles, mouse pads and the like. For example, the George Knight Company sells a transfer printing machine that has upper and lower platens for pressing transfer paper against textiles under heat and pressure to cause dyes from the paper to be transferred into the textile material.
An improved system is needed for transfer printing metal substrates to produce decorated sheets having consistent bright coloring with little or no texturing of the surface of the sheet. The process should produce decorated sheet metal to be formed into shaped articles such as appliance panels or three-dimensional pictures and signs without cracking or crazing of the coatings on the sheet metal. Transfer printing of metal substrates has heretofore frequently resulted in inconsistent and uneven transfer (mottling and/or ghosting) of the transferred images. Solid color areas have been difficult to produce in acceptable quality due to mottling. Ghosting is a faint offset second image which makes the product unacceptable. A process is needed for producing a sharp and consistent image in a variety of mass-produced products such as appliance panels, one-piece three-dimensional pictures and frames.